Spin filter device, method for its manufacture and its use

ABSTRACT

The present invention relates to a method and a device for providing a current of spin-polarised electrons. More particularly, the present invention is suited for use in spin electronics or detection of spin-polarised electrons.

CLAIM OF PRIORITY

This application claims the benefit of priority under 35 U.S.C. §119(a)-(d) of Great Britain Patent Application Serial Number 1101862.9,entitled “SPIN FILTER DEVICE, METHOD FOR ITS MANUFACTURE AND ITS USE,”filed on Feb. 3, 2011, the benefit of priority of which is claimedhereby, and which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a method and a device for providing acurrent of spin-polarised electrons. More particularly, the presentinvention is suited for use in spin electronics or detection ofspin-polarised electrons.

BACKGROUND

The generation of a current of spin-polarised electrons, in which allelectron spins point mostly in the same direction, is a central aspectof spin electronics (hereafter also called spintronics). Spintronics, orspin electronics, refers to the role played by electron spin in solidstate physics, and to devices that specifically exploit spin properties,i.e. spin degrees of freedom, instead of, or in addition to, chargedegrees of freedom. For example, spin relaxation and spin transport inmetals and semiconductors are of interest in fundamental research suchas solid state physics and more generally in other electronic technologyareas. Extensive research and development is carried out in the field ofspintronics with the objective to fully utilise the advantage that noelectron charges need to be transported which would cost energy andproduce heat.

To date, currents of spin-polarised electrons are predominantly obtainedfrom magnetic or magnetized materials. When the spin has to be invertedthe applied external magnetic field needs to be inverted. This istypically intrinsically slow to carry out, and would thus be inefficientfor many applications.

Furthermore, currents of spin-polarised electrons can be obtained whencircularly polarised light ejects electrons from substrates with largespin-orbit coupling, for example gallium arsenide (GaAs).

In the above mentioned methods, the materials used are prepared incomplex preparations under ultra-high vacuum (UHV) conditions. Thus,their integration into large scale integrated circuits or even printedcircuits is difficult.

An important attribute of free spin-polarised electrons is the degree ofpolarisation. The degree of polarisation of free spin-polarisedelectrons is presently measured by so called Mott scattering of highenergy electrons on thin gold foil under high vacuum conditions. Thehigh energy electrons are in the region of more than 50 kV and the thingold foil is in the region of a few nanometers. Alternatively, thedegree of polarisation of free spin-polarised electrons can be measuredby spin-dependent diffraction at surfaces of wolfram (W) or iron (Fe)under ultra-high vacuum conditions (less than 10⁻¹⁰ mbar). However, bothmethods for detecting spin-polarised electrons are technically complexand prone to errors.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a spin filter device with the features ofclaim 1, a method for manufacturing a spin filter device of claim 6 theuse of a spin filter device of claim 9 and a spintronic transistorstructure with the features of claim 15. Further dependent claimsdescribe preferred embodiments.

It is one object of the present invention to provide a method and systemwhich overcomes at least some of the problems relating to spin-polarisedelectrons.

It is thus an object of the present invention to provide a method anddevice for obtaining spin-polarised electrons which can be integratedand used in large scale integrated circuits, printed circuits and/orspintronic applications in general.

It is a further object of the present invention to provide a method anddevice for detecting spin-polarised electrons which can be employedwithout the need of complex and error-prone equipment.

It is a further object of the present invention to achieve highefficiency in spin selectivity.

According to an aspect of the present invention, there is provided aspin filter device including

-   -   a substrate and    -   at least one monolayer deposited upon said substrate;        wherein said monolayer comprises asymmetrical molecules, and is        adapted to filter electrons travelling from said substrate        through said at least one monolayer such that electrons that        exhibit a predetermined spin can pass.

Optionally, the molecules of said at least one monolayer are chiralmolecules. Chiral molecules lack mirror image symmetry and show twotypes of enantiomers that can be described as left-handed (L- or levo-)and right-handed (D- or dextro-) species. When a charge moves within achiral system in one direction it creates a magnetic field as a resultof so called broken mirror image symmetry.

Optionally, the substrate can be one of a metal or a semiconductor.

The substrate can optionally be polycrystalline or single crystalline.An example of the substrate suitable for the spin filter device of thepresent invention can include, but is not limited to, polycrystallinegold (Au) or single crystalline gold (Au(111)).

Optionally, the at least one monolayer can be self-assembled on thesubstrate, produced for example in a wet chemical procedure.

The said at least one monolayer can optionally comprise organicmolecules.

Optionally, the said at least one monolayer can comprise nano-particles,particularly nano-dots.

The molecules of the said at least one monolayer can optionally bethiolated molecules. An example of the thiolated molecules suitable forthe spin filter device of the invention can include, but is not limitedto, double stranded DNA. Double stranded DNA is chiral both because ofits primary structure and because of its secondary, double helixstructure. The molecules can have a predetermined length, e.g. thedouble stranded DNA can comprise for example 26, 40, 50, 78 or any othernumber of base pairs (bp) as considered appropriate for particularapplication of the present invention.

According to an aspect of the invention, there is provided a method formanufacturing a spin filter device, the method including the steps of

-   -   (a) providing a substrate;    -   (b) depositing at least one monolayer upon the substrate;        wherein the at least one monolayer comprises asymmetrical        molecules adapted to filter a current of electrons such that        electrons that exhibit a predetermined spin can pass to generate        a current of spin-polarised electrons.

The substrate and the at least one monolayer can include all or selectedfeatures of the substrate and the monolayer, respectively, and itspreferred embodiments as defined in relation to the aspect of theinvention further above in which a spin filter device is provided, asconsidered appropriate for manufacturing the spin filter device.

Step (a) of the method can optionally include cleaning the substrateprior to step (b), for example by sputtering with rare or reactive gasesor by wet chemical agents.

Optionally, the at least one monolayer can be deposited upon thesubstrate by self-assembling of the molecules. The self-assembling ofthe molecules can optionally be processed in a wet chemical procedure.

Optionally, step (b) of the method can include integratingnano-particles, particularly nano-dots, into the at least one monolayer.If there is more than one monolayer, the nano-particles can optionallybe integrated between the monolayers.

According to an aspect of the invention, there is provided a method forusing a spin filter device for generating a current having a firstsubstrate and at least one mono-layer deposited on the first substrate,the method including the steps of

-   -   (a) producing a current of electrons from the first substrate to        travel into the at least one monolayer; and    -   (b) injecting electrons which have passed the at least one        monolayer into a second substrate;        wherein the at least one monolayer comprises asymmetrical        molecules adapted to filter the current of electrons such that        electrons that exhibit a predetermined spin can pass to generate        a current of spin-polarised electrons.

Optionally, the first substrate is one of a metal or a semiconductor.The second substrate can optionally be one of a metal, a semiconductor,an isolator or a vacuum.

The at least one monolayer can include all or selected features of themonolayer and its preferred embodiments as defined in relation to theaspect of the invention in which a spin filter device is provided, asconsidered appropriate for using the spin filter device.

Optionally, step (a) of the method can include irradiating saidsubstrate with photons from a photon source thereby producing a currentof photoelectrons which travel into said at least one monolayer.

Optionally, step (a) of the method can include irradiating saidsubstrate with a UV laser pulse. Optionally, a photon energy of thephotoelectrons emitted by the UV laser can be 5.84 eV, wherein the pulseduration can be about 200 ps (picoseconds) at 20 kHz repetition rate,and a fluence of 150 pJ/cm². However, any other photon energy, pulseduration, repetition rate or fluence can be applied as appropriate. Thelaser pulse can optionally be generated from a Nd:YVO₄ oscillator.

Optionally, step (a) can be realised by electric or magnetic fieldinduced injection from a spin polarised substrate.

Step (a) can optionally include irradiating said substrate with linearlyor circularly polarised or unpolarised photons.

According to a further aspect of the present invention, there isprovided a spintronic transistor structure including a semiconductorstructure carrying a spin filter device including all or selectedfeatures of the spin filter device and its preferred embodiments asdefined in relation to the aspect of the invention in which a spinfilter device is provided, wherein the spin filter device is adapted tooperate as a spin injector the semiconductor structure.

Optionally, the semiconductor structure can comprise silicon or GaAs.

According to a further aspect of the present invention, there isprovided a detector for spin-polarised electrons, the detector includingat least one monolayer;

-   -   wherein the monolayer is deposited upon the detector such that        electrons to be detected have to pass the monolayer; and    -   wherein the monolayer comprises asymmetrical molecules and is        adapted to filter electrons such that electrons that exhibit a        predetermined spin can pass.

Optionally, the molecules of the at least one monolayer are chiralmolecules.

The said at least one monolayer can optionally comprise organicmolecules. Optionally, the at least one monolayer is self-assembled onthe detector, for example in a wet chemical procedure.

The at least one monolayer can optionally comprise nano-particles,particularly nano-dots.

An advantage of embodiments of the present invention is that magnetic ormagnetized substrates are no longer needed to generate spin-polarisedelectrons. Also, circularly polarised light is no longer needed togenerate spin-polarised electrons. The materials can be easily prepared,for example by wet chemical procedures, and have turned out to be stablein air. So, they can be used in room temperature and do not needexpensive cooling. A further advantage of the present invention is thatit provides an effective way to inject spin into standard transistors,such as silicon based transistor. The invention further enables spinsensitive gating. Furthermore the invention enables the reduction of theamount of space and heat capacitance of logic devices.

Another advantage of embodiments of the present invention arises fromthe fact that molecular scale electronics are used. This opens a way forincorporating the quantum mechanical spin concepts with a standarddevice such as those based on, for example silicon (Si) or galliumarsenide (GaAs). In molecular electronic devices, when trying to contactmolecules to the macroscopic world, problems can be circumvented withthe present invention. Embodiments of the present invention provide aviable alternative to the conventional schemes proposed for molecularelectronics.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described with reference toand as shown in the following Figures, in which

FIG. 1 is a perspective view of a spin filter device according to oneembodiment of the present invention;

FIG. 2 a shows a distribution of a spin polarisation percentage ofelectrons ejected from a clean substrate with linearly or circularlypolarised light according to the prior art;

FIG. 2 b shows a distribution of a spin polarisation percentage ofelectrons ejected from another bare substrate with linearly orcircularly polarised light according to the prior art;

FIGS. 3 a, 3 b, 3 c show a distribution of the spin polarisationpercentage of electrons transmitted through a spin filter deviceaccording to an embodiment of the present invention, for light polarisedclockwise (cw) circularly, linearly, and counter-clockwise (ccw)circularly, respectively;

FIG. 4 shows the electron spin polarisation for spin filter devicesaccording to the present invention and reference materials as a functionof the length of chiral molecules;

FIG. 5 shows a total photoelectron signal measured for electrons ejectedfrom a spin filter device according to the present invention applyingclockwise (cw) and counter-clockwise (ccw) polarised light;

FIGS. 6 a, 6 b, 6 c show a distribution of the spin polarisationpercentage of electrons transmitted through a spin filter devicecomprising a monolayer with molecules of a first length according to anembodiment of the present invention, for light polarised clockwise (cw)circularly, linearly, and counter-clockwise (ccw) circularly,respectively;

FIGS. 7 a, 7 b, 7 c show a distribution as in FIGS. 6 a-6 c but for aspin filter device comprising a monolayer with molecules of a secondlength according to an embodiment of the present invention;

FIGS. 8 a, 8 b, 8 c show another distribution as in FIGS. 6 a-6 c butfor a spin filter device comprising a monolayer with molecules of athird length according to an embodiment of the present invention;

FIG. 9 shows the spin dependent current through a monolayer of the spinfilter device according to an embodiment of the present invention;

FIG. 10 shows a spintronic transistor structure according to anembodiment of the present invention; and

FIG. 11 shows a spin filter device according to a further embodiment ofthe present invention.

DETAILED DESCRIPTION

FIG. 1 shows a perspective view of a spin filter device 10 according toone embodiment of the invention. In this example, the spin filter device10 includes a mono-layer 12 of double stranded DNA (dsDNA). ThiolateddsDNA is bound to a substrate 14, which is a gold surface in thisexample, and forms a self-assembled mono-layer 12 of chiral molecules.The substrate can also be formed by any other material, e.g. a differentmetal or a semiconductor. Unpolarised electrons 16 ejected from the goldsurface produced by linearly polarised light 18 incident from themonolayer side are transmitted and can be analyzed. Most of theelectrons 20 transmitted through the monolayer 12 are spin polarised.Spin orientation depends on the enantiomer used in the monolayer.

It is well known that spin-polarised photoelectrons are readilygenerated from magnetic substrates or when circularly polarised lightejects electrons from substrates with large spin-orbit coupling. Sincean organic chiral layer on a non-magnetic metal surface is not likely tobe self-magnetized, one expects that photoelectrons ejected from such alayer with unpolarised light will not be spin polarised. The presentinvention however shows exceptionally high polarisation of electronswhich are ejected from surfaces coated with self-assembled monolayer ofdouble stranded DNA, independent of the polarisation of the incidentlight. It has previously been shown that the photoelectron yield fromself-assembled monolayers of chiral molecules on gold depends on thecircular polarisation of the exciting light as well as the voltageacross the layer and its handedness. The spin polarisation of theelectrons was not measured, and indications for spin-dependenttransmission were only inferred from the dependence of the totalelectron yield on the circular polarisation of the incident photons.These studies could not determine whether or not the ejected electronsare spin polarised when the incident photons are unpolarised or linearlypolarised. Furthermore, the observed effect may result from circulardichroism, namely that the absorption of the system depends on thelight's circular polarisation.

In the present invention, self-assembled dsDNA monolayers can beprepared according to standard procedures by depositing dsDNA which isthiolated on one, e.g. the 3′, end of one of the DNA strands on a cleangold substrate. Either polycrystal-line Au or single crystal Au(111) maybe used as substrates. The monolayers are characterized by variousmethods that ensure the uniformity and reproducibility of the DNA layer.The experiments have been carried out under ultra-high vacuum conditionsthat employed photoelectron detection with two detectors, an electrontime-of-flight instrument, recording the kinetic energy distribution ofthe electrons, and a Mott-type electron polarimeter for spin analysis.The photoelectrons were ejected by a UV laser pulse with a photon energyof 5.84 eV, pulse duration of about 200 ps at 20 kHz repetition rate,and a fluence of 150 pJ/cm². The laser light is normally incident ontothe sample, and it is either linearly or circularly polarised. For thevast majority of DNA samples no damage is observed over the course ofthe spin polarisation measurement within about four hours. For directpolarisation measurements, the photoelectrons are guided by anelectrostatic 90°-bender and subsequent transport optics. Hence, aninitial longitudinal spin polarisation is converted into a transversalone for analysis. In the electron polarimeter, an electron spinpolarisation causes an up-down scattering asymmetryA=(I_(U)−I_(L))/(I_(U)+I_(L)). Here I_(U,L) denote the count rates of anupper and a lower counter. The transversal polarisation is given byP=A/S_(eff). The analysing power, also known as the Sherman function,has been calibrated to be S_(eff)=−(0.229±0.011). In the above set-up,the spin polarisation parallel to the sample normal and thus parallel tothe initial electron velocity is measured.

FIG. 2 a and FIG. 2 b illustrate examples of measurements ofspin-polarisation with prior art devices. In FIG. 2 a, the spinpolarisation of photoelectrons from a clean Au(111) single crystal isshown for different laser polarisations. The spin polarisation and thesign of its orientation depend on the laser polarisation. The comparisonof the counting rates in the upper and lower counter of the Mottpolarimeter yields an intensity asymmetry of A=±(5.03±1.1) % forclockwise (cw, curve 22) and counter clockwise (ccw, curve 24)circularly polarised light. Combined with the Sherman function S_(eff)average electron spin polarisations of P=±(22±5) % are determined foremission from the single crystal substrate, shown by the histograms 26,28. For negative electron spin polarisation, the direction of the spinvector is antiparallel to the propagation direction of the electron, andcorrespondingly parallel to {right arrow over (k)} for positive spinpolarisation. As expected for an unmagnetized substrate, no asymmetryand therefore also no spin polarisation is observed for electronsejected from the gold surface when the laser is polarised linearly(curve 30, histogram 32). For reference, the spin polarisation is shownfor electrons emitted from the molybdenum sample holder in FIG. 2 b. Inthis case, the spin polarisation is zero for circular as well as linearlaser polarisation (curve 34). This signal has been used as a referencefor the further experiments.

FIGS. 3 a, 3 b and 3 c illustrate examples of measurements ofspin-polarisation of spin filter device as shown in FIG. 1 according toan embodiment of the present invention. More specifically, FIGS. 3 a, 3b and 3 c show the spin polarisation observed when electrons from thesame Au(111) single crystal are transmitted through a self-assembledmonolayer of 50 base pairs (bp) dsDNA. Excitation with linearlypolarised light (FIG. 3 b) produces now strongly spin polarisedelectrons with an average P=−(31±4) % (curve 36). Also, excitation byclockwise (cw) (FIG. 3 a) and counter-clockwise (ccw) (FIG. 3 c)circularly polarised light yields spin polarised electrons with averagepolarisations of P=−(35±3) % and −(29±3) % (curves 38, 40),respectively. All electron spin polarisations show the same negativesign, independent of the light polarisation. A similar high spinpolarisation is observed for linearly polarised light too (FIG. 3 b,curve 36). These results stand in contrast to the case of the clean Au(111) substrate as shown in FIG. 2 a, for which linearly polarised lightinduces no electron spin polarisation (curve 30), and for which thehelicity of the circularly polarised light determines the direction ofthe electron spin polarisation. These results indicate that the orderedmonolayer 12 (FIG. 1) of dsDNA acts as a spin filter for electrons 16,20 (FIG. 1) excited in the gold as substrate 14 (FIG. 1) and transmittedthrough the dsDNA monolayer 12.

FIG. 4 shows measurements of the electron spin polarisation for variousmonolayers 12 (FIG. 1) of DNA on polycrystalline gold (Au) surfaces.More specifically, room temperature electron spin polarisations arepresented for four different monolayers of dsDNA in which each of themolecules have different lengths of 26, 40, 50, and 78 base pairs (linewith filled dots shown at 42 a,b,c, 44 a,b,c, 46 a,b,c, 48 a,b,c). Theresults of 36 different experiments on ten different samples are shown.Further, monolayers of single stranded DNA (lines with open diamondshown at 50 a,b,c), and a sample of dsDNA damaged by UV light (lineswith open triangle shown at 52 a,b,c) are investigated. Also theelectron spin polarisation from an un-cleaned bare polycrystalline goldsubstrate (open circle 54 a,b,c) is shown at zero base pairs.Measurements are conducted with light of different polarisations, cw (42a, 44 a, 46 a, 48 a, 50 a, 52 a, 54 a) and ccw (42 b, 44 b, 46 b, 48 b,50 b, 52 b, 54 b) circular, and linear (42 c, 44 c, 46 c, 48 c, 50 c, 52c, 54 c). For clarity, the symbols for the different light polarisationsare off-set by a plus and minus one base pair. The data indicates thatwith increasing length of the dsDNA molecules the absolute value ofelectron spin polarisation increases. The highest electron spinpolarisation is observed for 78 bp dsDNA with about P=−60% (shown at 48a,b,c). While for dsDNA on average the electron spin polarisationincreases with the length of the DNA, no polarisation is obtained forsingle stranded DNA (shown at 50 a,b,c). In an absolute value, thepolarisation increases slightly in the case of dsDNA, when the electronsare injected with ccw circular polarised light (42 b, 44 b, 46 b, 48 b),because there are more electrons injected into the monolayer 12 with aspin polarisation that coincides with a high transmission of themonolayer 12. Given error bars are standard deviation of the mean ofseveral runs, namely single measurements each having a statistical errorof about less than 3%. Measurements performed on different samplescoated with the same number of base pairs are averaged and the totalerror is calculated by error propagation.

The measurements presented in the Figures as described above indicatethat well-organized self-assembled monolayers 12 of dsDNA on Au assubstrate 14 act as very efficient spin filters. Within the range ofdsDNA length studied, the selectivity increases with its length andtherefore the number of turns of the helix. It is important toappreciate that even the longest molecules used are still shorter thanthe persistence length of the DNA, which is the length up to which theDNA behaves as a rigid rod. Hence, the dsDNA oligomers studied here arerigid and each monolayer 12 can be visualized as consisting of rigidchiral rods closely packed together, as depicted in FIG. 1. In the caseof a monolayer made from single strand DNA, the molecules are morefloppy and do not form rigid close-packed monolayers and indeed no spinselectivity is observed. Because the photon energy is lower than theionization energy of the DNA and the laser intensity is low, thephotoelectrons all originate from the gold substrate. In addition, lessthan 0.1% of the incident light is absorbed in the layer, even underresonance conditions. The low intensities and weak absorbances ensurefurther that non-linear excitation processes do not occur in the DNAlayer.

Electrons are known to transmit through free standing or supported thinferromagnetic films that acted as a spin filter in certain situations.In these cases, and for low-energy electrons, the selectivity wasreported to be about 25%. The spin polarisation can be explained byinelastic electron scattering involving unoccupied d-states above theFermi level. The scattering rate for minority spin electrons is thenenhanced with respect to that of majority spin electrons because of anexcess of minority spin holes. The polarisation decreased sharply as afunction of collision energies, due to the spin dilution by secondaryelectrons. However, in the present invention the polarisation is energyindependent within the energy range studied. Although polarised light isnot needed, the polarisation achieved with embodiments of the presentinvention is almost as high as that obtained by photoemission withcircular polarised light from GaAs substrates.

The mechanism of how charge transport or charge redistribution throughchiral systems generates a magnetic field is elementary; however, thismagnetism is transient, ending when the charge flow stops. A possibleway to transform transient charge flow into permanent magnetism is byspin-orbit coupling that converts the orbital angular momenta of theelectrons in the helical potential into spin alignment. Spin-orbitcoupling in hydrocarbons is commonly believed to be very weak andtherefore no significant spin alignment is expected. Indeed, theinteraction of spin polarised electrons with chiral molecules hasearlier been studied. When these electrons were scattered from gas phaseand thus randomly oriented chiral molecules, only a very smallpreference of the order or 10⁻⁴ of one spin orientation over the otherwas found, and only when a heavy metal atom with significant spin-orbitinteraction was present in the molecules. In contrast to these gas phasestudies, electrons transmitted through organized monolayers ofdipolar-chiral molecules of the present invention display a largedependence on the handedness of the molecules.

FIG. 5 shows the kinetic energy distribution of the electrons 16, 20(FIG. 1) transmitted through the self-assembled monolayer 12 (FIG. 1) ofdsDNA (50 bp) adsorbed on a polycrystalline Au substrate 14 (FIG. 1) forclockwise (cw, curve 56) and counter clockwise (ccw, curve 58)circularly polarised light. Photoelectrons with kinetic energies up toabout 1.2 eV are measured. The intensity of the signal (curve 58)observed with ccw circularly polarised radiation is larger by about 7%relative to that obtained with the cw circularly polarised light (curve56). This ratio is independent of the primary kinetic energy of thephoto-emitted electrons. This result clearly shows a circular dichroismwhich is consistent with the results obtained in FIG. 2 a. In thismeasurement the electron signal is not spin resolved. It further showsthat the work function is independent of the handedness of the light, asexpected.

In FIGS. 6 a, 6 b, 6 c, 7 a, 7 b, 7 c, 8 a, 8 b and 8 c, typical resultsfor one preparation are shown for 40 bp, 50 bp, and 78 by monolayers 12(FIG. 1) of dsDNA on poly-Au substrates 14 (FIG. 1).

FIGS. 6 a, 6 b and 6 c show measurements of the spin polarisation of 40bp dsDNA/poly-Au. The spin polarisation is −(38.0±6.5) % for linearpolarised light (FIG. 6 a), −(35.1±8.3) % for cw polarised light (FIGS.6 b), and −(40.1±5.5) % for ccw polarised light (FIG. 6 c).

FIGS. 7 a, 7 b and 7 c show measurements of the spin polarisation of 50bp dsDNA/poly-Au. The spin polarisation is −(35.5±5.5) % for linearpolarised light (FIG. 7 a), −(31.5±5.7) % for cw polarised light (FIGS.7 b), and −(38.8±5.9) % for ccw polarised light (FIG. 7 c).

FIGS. 8 a, 8 b and 8 c show measurements of the spin polarisation of 78bp dsDNA/poly-Au. The spin polarisation is −(57.2±5.9) % for linearpolarised light (FIG. 8 a), −(54.5±7.0) % for cw polarised light (FIG. 8b), and −(60.8±5.8) % for ccw polarised light (FIG. 8 c).

FIG. 9 shows the spin dependent current through double stranded DNAwhich is 40 base pairs long. The current of electrons with spin up isshown in curve 60, whereas the current of electrons with spin down isshown in curve 62. At ±1 Volt the ratio between the two spin currents60, 62 is 1:5. These measurements show that trans-port through thesemolecules favours one spin orientation.

FIG. 10 shows a spintronic transistor structure 70 according to anembodiment of the invention. It comprises three monolayers 12 of organicmolecules and nano-dots 72 incorporated between the monolayers 12. Themonolayers 12 are self-assembled to the substrate 14 which is made ofgold (Au). The spintronic transistor structure 70 is based on asemiconductor structure 74 comprising GaAs. The transistor 70 can creatediscrete gating for non binary logic operation. By creating severalplateaus-like features in the IV characteristics of the transistors 70 areduction of the power of standard transistors working in non binarybase may be obtained.

In FIG. 11, a spin filter device 80 is shown as an example for a furtherembodiment according to the present invention. A first substrate 82,e.g. a metal or a semiconductor, acts as source material. A monolayer 86of asymmetrical molecules, preferably organized chiral molecules isdeposited upon the first substrate 82. The first substrate 82 and themonolayer 86 form the spin filter device. A second substrate 84, whichcan be for example a metal, a semiconductor, an isolator or a vacuum,acts as target material. The monolayer 86 is located between the firstand the second substrate 82, 84. Preferably, the monolayer 86 isdeposited upon or bounded to the first substrate 82 by self-assembling.Electrons (not shown) travel from the first substrate 84 into themonolayer 86. The majority of those electrons which have passed themonolayer 86 have the same spin. These spin filtered or spin-polarisedelectrons are injected into the second substrate 84.

The sample organisation for the high spin selectivity is important.Measurements further provide quantitative information regarding the spinpolarisation and its dependence on the monolayer thickness or the lengthof the helical potential. If the effect described in relation to theFigures is caused by a pseudo-magnetic field within the monolayer itmeans that a field exceeding a few hundred Tesla has to be present.

The present invention provides for practical and theoreticalconsiderations allowing for configuring a novel spin filter device thatcan be used in spintronic devices. This structure is characterized byspin selectivity for electron transmission therethrough. The spin filterdevice of the present invention can be used in a spintronic transistorstructure.

Those skilled in the art can appreciate that while the present inventionhas been described in terms of preferred embodiments, the concept uponwhich the invention is based may readily be utilized as a basis fordesigning other structures, systems and processes for carrying out theseveral purposes of the present invention.

In terms of the monolayer 12, it is appreciated that other kinds ofasymmetrical and/or chiral molecules than double stranded DNA can beused to achieve the present advantages.

Although the examples of utilization of the spin filter device 10 of thepresent invention were shown for a spin filter device 80 and as a partof a spintronics circuit, e.g. in large scale integrated circuits orprinted circuits, the structure can also be used as components in otherdetectors or sensors.

It is important, therefore, that the scope of the invention is notconstrued as being limited by the illustrative embodiments describedherein. Other variations are possible within the scope of the presentinvention as defined herein.

1. A spin filter device, the device comprising: a substrate; and atleast one monolayer deposited upon said substrate, wherein saidmonolayer comprises asymmetrical molecules, and is adapted to filterelectrons travelling from said substrate through said at least onemonolayer such that electrons that exhibit a predetermined spin canpass.
 2. The spin filter device of claim 1, wherein the molecules ofsaid at least one monolayer are chiral molecules.
 3. The spin filterdevice of claim 1, wherein the substrate can be one of a metal or asemiconductor, wherein the substrate is preferably polycrystalline orsingle crystalline.
 4. The spin filter device of claim 1, wherein the atleast one monolayer is self-assembled on the substrate, preferablyproduced in a wet chemical procedure.
 5. The spin filter device of claim1, wherein the at least one monolayer comprises organic molecules,preferably thiolated molecules and more preferably double stranded DNA,or nano-particles, preferably nano-dots.
 6. A method for manufacturing aspin filter device of claim 1, the method comprising: (a) providing asubstrate; and (b) depositing at least one monolayer upon the substrate;wherein the at least one monolayer comprises asymmetrical moleculesadapted to filter a current of electrons such that electrons thatexhibit a predetermined spin can pass to generate a current ofspin-polarised electrons.
 7. The method of claim 6, wherein step (a) ofthe method includes cleaning the substrate prior to step (b), preferablyby sputtering with rare or reactive gases or by wet chemical agents. 8.The method of claim 6, wherein step (b) of the method includesintegrating nano-particles, particularly nano-dots, into the at leastone monolayer, or, if there is more than one monolayer, thenano-particles are integrated between the monolayers.
 9. Use of a spinfilter device for generating a current having a first substrate and atleast one monolayer deposited on the first substrate, with the followingsteps of (a) producing a current of electrons from the first substrateto travel into the at least one monolayer; and (b) injecting electronswhich have passed the at least one monolayer into a second substrate;wherein the at least one monolayer comprises asymmetrical moleculesadapted to filter the current of electrons such that electrons thatexhibit a predetermined spin can pass to generate a current ofspin-polarised electrons.
 10. Use of claim 9, wherein the firstsubstrate is one of a metal or a semiconductor and/or the secondsubstrate is one of a metal, a semiconductor, an isolator or a vacuum.11. Use of claim 9, wherein the molecules of said at least one monolayerare chiral molecules.
 12. Use of claim 9, wherein the at least onemonolayer is self-assembled on the substrate, preferably produced in awet chemical procedure.
 13. Use of claim 9, wherein the at least onemonolayer comprises organic molecules, preferably thiolated moleculesand more preferably double stranded DNA, or nano-particles, preferablynano-dots.
 14. Use of claim 9, wherein step (a) includes irradiating thesubstrate with photons from a photon source thereby producing a currentof photoelectrons which travel into the at least one monolayer,preferably with an UV laser pulse, more preferably an UV laser pulsegenerated from a Nd:YVO₄ oscillator, and/or step (a) includesirradiating the substrate with linearly or circularly polarised orunpolarised photons.
 15. A spintronic transistor structure, saidstructure comprising: a semiconductor structure carrying a spin filterdevice of claim 1, wherein the spin filter device is adapted to operateas a spin injector for the semiconductor structure.
 16. The spintronictransistor structure of claim 15, wherein the semiconductor structurecomprises silicon or GaAs.